1. Field of the Invention
The present invention is directed to methods for estimating corrections for the image degradation produced in medical ultrasound images by phasefront aberrations and reverberations. The method hence has applications to all situations were ultrasound imaging is used in medicine, and also other similar situations of ultrasound imaging.
2. Description of the Related Art
With ultrasound imaging of objects through complex structures of tissue, the following effects will degrade the images
i) Variations of the acoustic velocity within the complex tissue structures produce aberrations of the acoustic wavefront, destroying the focusing of the beam mainlobe and increasing the beam sidelobes.*
ii) Interfaces between materials with large differences in acoustic properties can give so strong reflections of the ultrasound pulse that multiple reflections get large amplitudes. Such multiple reflections are termed pulse reverberations, and add a tail to the propagating ultrasound pulse, which shows as noise in the ultrasound image.*
The reduced focusing of the beam main lobe reduces the spatial resolution in the ultrasound imaging system. The increase in beam side lobes and the pulse reverberations, introduce additive noise in the image, which is termed acoustic noise as it is produced by the transmitted ultrasound pulse itself. Increasing the transmitted pulse power will hence not improve the power ratio of the signal to the noise of this type, contrary to what is found with electronic receiver noise.
The materials with largest differences in acoustic properties are muscles, fat, connective tissue, cartilage, bone, air, and the ultrasound transducer itself. Mixtures of fat, muscles, connective tissue and cartilage in the body wall can therefore produce very large phase front aberrations and reverberations. Especially, one will get strong reverberations from the transducer reflections of the returning signals from interfaces of such tissues in the body wall. The mixtures of such tissues in the body wall are therefore the major cause of the degradation found with non-invasive ultrasound imaging in many patients. Reducing the effect of the reverberations and phase front aberrations in the body wall is hence much needed in many applications of medical ultrasound imaging.
With a two-dimensional transducer array, the effect of the phasefront aberrations can in many situations be reduced by adding corrective delays and gain factors to the signals for the individual array elements, in the following referred to as element signals. This has been presented in many papers. In more complex situations of tissue mixtures, the phasefront aberrations and pulse reverberations can produce modifications of the pulse form. It is less known that such pulse modifications can be corrected by a filter for each of the element signals. Such correction filters gives the most general correction method, and delay/amplitude corrections can be considered as a special case or an approximation of correction filters.
2nd harmonic ultrasound imaging reduces the body-wall reverberations and the phasefront aberrations. The basis for this method is that the wave propagation velocity increases with the pressure amplitude, so that the positive pressure swing gets a higher propagation velocity than the negative pressure swing. This produces a non-linear propagation distortion of the forward propagating pulse that increases with the transmitted pulse amplitude. The distortion first increases with the propagation depth, producing higher harmonic frequency bands in the pulse oscillation. However, because of the power absorption of ultrasound in tissue increases with frequency, the distortion reaches a maximum and finally reduces for large depths.
Transmitting a pulse with center frequency around f0≈1.5 MHz, one gets adequate power in the 2nd harmonic band around 2f0≈3 MHz for imaging of the heart and other organs in the 3-15 cm range. The power in the higher than the 2nd harmonic component is so low that it is not useful for imaging.
The 2nd harmonic imaging has two advantages above first harmonic imaging around the same receive frequency:
i) The 2nd harmonic amplitude is very low as the outbound pulse passes the body wall, so that the 2nd harmonic components in the body-wall reverberations are low.*
ii) As the transmitted frequency f0 is low, the first harmonic transmitted beam is less affected by phase aberrations in the body wall. The 2nd harmonic beam is also generated over a certain volume, which makes the 2nd harmonic beam less sensitive to phase aberrations, and the field past the focus is also more collimated than for a 1st harmonic beam at 2f0.* These two effects hence produce less body-wall reverberations and phase aberrations in the 2nd harmonic pulse compared to the 1st harmonic pulse at the same frequency.
However, when the back-scattered signal passes the body wall on its return, the 2nd harmonic components are subject to the same amount of phasefront aberrations and pulse reverberations as the 1st harmonic pulse. Corrections for phasefront aberrations and pulse reverberations in the receive beamformer, hence improves the receiver resolution and acoustic noise, in the same way as for the 1st harmonic image. Also, there will be some residual phasefront aberrations and body-wall reverberations in the transmitted 2nd harmonic pulse, which can further be reduced by corrections in the transmit beam former.
Although the principle of correction for phasefront aberrations and pulse reverberations is well defined, it is in the imaging situation generally difficult to determine the correction filters or the simplified delay and amplitude corrections. The present invention devices two solutions to this problem:
i) The backscattered signal from point scatterers that are artificially introduced into the body serves as beacons to determine the correction filters. Such point scatterers are either connected to intervention tools that are introduced into the body, for example a biopsy needle, or ultrasound contrast agent bubbles in such a dilute concentration that the signals from individual bubbles can be discriminated from each other. The point scatterers must be so spaced apart that the signals from different point scatterers are clearly differentiable from each other and be so strong that they are differentiable from the tissue signal. To maintain adequate distance between the bubbles, the invention devices to use high transmitted pulse amplitudes to destroy the bubbles in selected image regions, so that for an adequate interval of time after this bubble destruction, new inflow bubbles have adequate distance to each other. Consecutive transmission of high amplitude pulses into the region can be used for repeated destruction of the bubbles so that a continuously changing set of point scatterer bubbles in the image region is obtained. To discriminate the bubble signal from tissue signal one can typically use backscattered frequency components in a band around the the 3rd or 4th harmonic component of the transmit frequency band, or sub-harmonic components. Transmission of coded sequences with pulse compression of the received signal can also be used to improve the signal to noise ratio in the received signal from the contrast agent bubbles, and hence the detection of the signal. Spaced apart contrast agent bubbles can also be used as point scatterers on the intervention tool.*
ii) With the other method one uses stochastic analysis of the back-scattered signal from distributed scatterers with short correlation length compared to the wave length. One general problem for such analysis is that the backscattered signal is corroborated with acoustic noise from pulse reverberations and phase front aberrations, and the invention devices two methods for reduction of such acoustic noise before the determination of the correction filters:
a) The 2nd harmonic component of the backscattered element signals, which has reduced acoustic noise, is used for the analysis. However, this requires one filter per element signal, and the invention therefore also devices a simplified method of using the 2nd harmonic component of the backscattered signal, where filtering of the individual element signals is avoided. In this method, the element signals are compared with a reference signal obtained from the element signals, the reference signal being modified so that the 1st harmonic band in the reference signal is highly attenuated, for example through filtering or pulse inversion techniques that are commonly known.*
b) The body wall pulse reverberations are fairly stationary in time. By using the backscattered signal from moving or time varying scatterers obtained with multiple transmit pulses with the same focus and beam direction, the temporally stationary acoustic noise is suppressed by highpass filtering each range in the backscattered signal along the pulse number coordinate, so that mainly the signals from the moving scatterers passes the filter for further processing. Typical moving scatterers can be the myocardium or an arterial wall, or scatterers found in blood or other body fluids. To enhance the scattering from such fluids, the invention also devices the use of ultrasound contrast agent to be injected into the body fluid. Time varying scatterers can be ultrasound contrast agent where so high transmit pulses are used that one get destruction of at least some of the contrast agent bubbles between the pulses.*
The stochastic analysis commonly contains an averaging of signal parameters, where averaging of signal parameters from different depths or possibly also different beam directions typically can be used. Such methods often provides a limited number of samples to average, which gives variance noise in the estimates.
To improve the estimation robustness and reduce the variance in the estimates, the invention devices a method that uses the backscattered signal from moving or time varying scatterers acquired with multiple transmit pulses with the same focus and beam direction. Signal parameters obtained for each transmit pulse are then averaged for many transmit pulses, possibly in combination with averaging over depth and beam direction, to reduce the variance in the estimates.
In addition to these basic principles, the invention devices several detailed methods for estimation of the correction filters.
Strong scatterers off the beam axis, can introduce interference in the correction estimates, and the invention devices methods to reduce the effect of such scatterers, using spatial lowpass filtering of the received signal across the transducer surface, or highpass filtering of estimated correction phases or delays across the transducer surface. Such highpass filtering can conveniently be done by expanding the correction delays in a generalized Fourier series, for example using Legendre polynomials as basis functions, and leaving out the lowest coefficients that relates to offset direction and possible offset focusing of the scatterer.
The correlation length of the phase aberrations and pulse reverberations along the transducer array surface has a lower bound. One can therefore also truncate the generalized Fourier series at the upper end, reducing the total number of coefficients in the series. The information carrying coefficients in the generalized Fourier series is hence a reduced parameter set that represents the correction filters, and is conveniently estimated in a parameter estimation scheme.
Often one also find that the correlation length of the phase aberrations and pulse reverberations along the transducer array surface is larger than the array element dimensions, as for example with phased arrays for sector steering of the beam. For this situation the invention devices combination of the element signals from neighboring elements before estimation of the correction filters. This combination reduces the total number of signal channels used in the estimation, hence simplifying the processing and increasing the signal to noise ratio in the resulting channel.